1. Field of the Invention
The present invention relates to an image-taking optical system and more particularly, it relates to a high-performance and compact image-taking optical system which is suitable for a digital device (a digital still camera, a digital video camera and the like) which takes an image of an object by a solid-state image pickup element.
2. Description of the Background Art
Recently, as a personal computer or the like is diffused, a digital still camera, a digital video camera and the like (referred to as the “digital camera” simply hereinafter) which can easily retrieve an image information into a digital device has been spread to a personal user. Thus it is expected that the digital camera is increasingly spread as an input device of an image information in the future.
In addition, the solid-state image pickup element such as a CCD (Charge Coupled Device) which is mounted on the digital camera has been miniaturized and accordingly it is required that the digital camera is further miniaturized. Therefore, it is earnestly desired that the image-taking optical system occupying a maximum volume in the digital device is made compact. In order to miniaturize the image-taking optical system, it is the easiest method to reduce a size of the solid-state image pickup element. However, in this case, it is necessary to reduce a size of a light-receiving element, and it becomes difficult to manufacture the solid-state image pickup element and performance required for the image-taking optical system becomes high.
However, when the size of the image-taking optical system is reduced while the size of the solid-state image pickup element is as it is, an exit pupil position consequently comes close to an image surface. When the exit pupil position comes close to the image surface, since off-axis beam emitted from the image-taking optical system is obliquely inputted to the image surface, a light converging performance of a micro lens provided on a front surface of the solid-state image pickup element is not efficiently provided, so that luminance of the image in an image center part is extremely different from that of the image in an image peripheral part. In order to solve the above problem, when the exit pupil position of the image-taking optical system is provided apart from the image surface, the entire image-taking optical system inevitably becomes large.
In addition, because of a recent low-price competition, a low-cost image-taking optical system has been strongly required. In addition, as a density of the solid-state image pickup element becomes high recently, the performance required for the image-taking optical system becomes high. By the above requirements, an image-taking optical system for a solid-state image pickup element comprising three lenses in which an aperture stop is provided in front of the lenses is disclosed in Japanese Unexamined Patent Publication No. 2-191907, Japanese Unexamined Patent Publication No. 4-153612, Japanese Unexamined Patent Publication No. 5-188284, Japanese Unexamined Patent Publication No. 9-288235, or U.S. Pat. No. 6,466,377.
However, according to the three-lens constitution disclosed in the Japanese Unexamined Patent Publication No. 2-191907, the Japanese Unexamined Patent Publication No. 4-153612,the Japanese Unexamined Patent Publication No. 5-188284, or the Japanese Unexamined Patent Publication No. 9-288235, negative power of the second lens is strong. Therefore, manufacturing error sensitivity to a lens axis becomes extremely high, so that it is likely to become difficult to maintain a periphery performance at the time of manufacturing. In addition, according to the three-lens image-taking optical system disclosed in the U.S. Pat. No. 6,466,377, a negative meniscus lens having a convex surface on an object side is provided as the second lens. Therefore, a beam angle inputted to the second lens on the image surface side becomes too steep, which could be totally reflected.